1. Field of the Invention
The present invention relates to a signal control system in a time division switching system, and more particularly to the arrangement of a speech path memory of a time switch in the time division switching system and a system for controlling a speech signal and a non-speech signal which pass through the speech path memory.
2. Description of the Prior Art
In a digital circuit switching system, speech path equipment of the TST (Time Switch-Space Switch-Time Switch) arrangement is often provided between line modules, each having accommodated therein many subscriber line circuits and trunk circuits (See Nikkei Electronics, June 25, 1979, pp.56-61). In the line module on the output side (which recei the output from a telephone or line), the subscriber line circuits are multiplexed into one highway. The data transmitted over this highway are such that each frame consists of a predetermined number of channels, for example, 128 channels. In each frame of the data, speech data necessary for a call, such as, for example, sound data, and non-speech data necessary for line control, such as, for example, on-hook data, are respectively assigned to predetermined channels. The line module on the input side (which applies a signal to a telephone or line) separates and distributes the data from one highway to many subscribers.
During data transmission and reception between the line modules, it is general practice in the prior art to extract the non-speech data from the data transmitted over the highway from the line module of the output side, to input only the speech data to a speech path memory in a time switch of speech path equipment and to apply the non-speech data to an exclusive non-speech path memory different from the speech path memory. Also in the line module of the input side, the speech data from the speech path memory in the time switch and the non-speech data from the exclusive non-speech path memory are inserted into one highway for input to the line module for the input side.
In the prior art, since data on the lines are concentrated and fed to the speech path memory, the memory has a small capacity and has no addresses corresponding to the lines; consequently, a signal representing on-hook, off-hook or a like state for each line and other non-speech signals cannot be transmitted via the speech path memory.
Accordingly, the prior art device is disadvantageous in that it needs a memory for the exclusive use of the non-speech data and hence is expensive. In addition, it is necessary to control the speech data and the non-speech data separately from each other, which inevitably introduces complexity in the control of data transmission and reception. Further, it has heretofore been proposed, for non-speech data processing, that the line module be divided into a plurality of units (for example, a subscriber line concentration unit, a trunk circuit multiplexing unit and so forth) according to the kinds of lines accommodated therein. It also has been proposed that a non-speech data processor be incorporated in each unit for processing the non-speech signal to some extent, thereby being able to interface with a switch module using a uniform format. This permits effective use of non-speech channels on the highway, by which it is possible to flexibly accommodate a variety of line modules. Such a conventional method is, however, defective in that when the line module is used as the line concentration unit, the number of lines accommodated is limited by a concentration ratio. Because the number of lines accommodated in one line module is small, the line concentration ratio is low. In addition, a division loss causes an increase in the overhead of software for switching signal control in a small-scale system and, thus raises the cost thereof.
Moreover, the prior art encounters a problem in signal transmission and reception between a telephone exchange and other equipment provided outside the switching system. For example, master and fireman's stations connected with a volunteer fire unit can normally be used as individual telephones. When the master station transmits urgent or high priority information to some or all of the fireman stations an exchange having received a special number transmitted from the master station, after originating a call, must transfer to the volunteer fire unit a control signal used for calling the required fireman's stations at the same time. Also it is necessary to transfer to the exchange a control signal for supervising the urgent traffic between the master station and the fireman stations. The control signal is control information that does not correspond to each subscriber and which has a larger amount of control information than does a general line circuit control signal. With one of the signal control systems heretofore employed, however, such a special control signal must also be transferred within the frame length range of such a limited transmittable amount of information as mentioned previously (64 bits per frame). It is necessary in some cases to take a step of, for example, using, as the control signal, control information or frames which sometimes become unused because of non-connection of telephones with a plurality of line circuits. There is a tendency for special terminal equipment requiring such a control signal having a large amount of information to gradually spread or increase in number within a telephone company; hence, each time such special terminal equipment is employed, it is necessary to work out special measures for transferring the required control signal. It is especially difficult to accommodate this information not corresponding to each subscriber and each line.
According to one of the conventional speech path control systems, the configuration of a digital highway interconnecting the line module and the switch module is fixed or limited to 120 speech paths and eight signal paths. The eight signal paths are connected by a dropper and an inserter to a main processor via a scanner and a signal distributor without being directly connected to the speech path memory of the time switch in the switch module. Therefore, when no control by the main processor of the exchange is required, as in the case of special (pulse coded modulation) PCM lines which are accommodated in the line module and from which control signals have been separated by a common signalling system, digital signals must be transmitted over the 120 speech paths alone, leaving the eight signal paths unused. In this respect the prior art speech path control system is economically disadvantageous.
As described above, it is customary in the prior art to process the speech data and the non-speech data (signals, control information and so forth) separately from each other and to concentrate the speech data. Accordingly, the prior art possesses shortcomings in that complex control is needed because the lines and the data (the speech data and the non-speech data) have no one-to-one correspondence and in that the flexibility is lost with respect to the kinds of line modules that can be used. In addition, future enlargement of services when employing uniform interfaces for the highway between the line modules and the switch module is impaired, and limitations are imposed on the arrangement of the line modules which make it uneconomical. On the other hand, the subscriber line circuit has markedly been improved by LSI technology in recent years and, as a result, it becomes easy to perform concurrent processing of the non-speech data and the speech data on the telephone or line side of the line module. This will permit direct transmission and reception of control information through the speech channel, which will enable introduction of new services. But the prior art is economically disadvantageous, especially so in the case of small-scale systems.